The conventional version of this type of pulse encoder comprises a light emitting diode or other light emitting unit, a light receiving element unit having a PN type photodiode or other light receiving element for receiving light from the light emitting unit, a rotary encoder disc and a fixed encoder disc having signal patterns which pass or interrupt light sent from the light emitting unit to the light receiving element unit. A fixed slit plate has the fixed encoder disc and the light receiving element unit affixed. A housing has the fixed slit plate affixed, and a printed circuit board on which is formed the signal processing circuit (comparator, driver, TTL, etc.) to which the signal output from the light receiving element unit is input through connection lines.
However, in this type of pulse encoder, the signal output from the light receiving element unit is on the high order of several hundred mV, so the signal, in the process of passing from the light receiving element to the amplifier on the printed circuit board (for example, the comparator) is particularly susceptible to noise. Therefore, the signal output to the outside of the pulse encoder often is not correct and thus mistaken operations are caused.
However, the major reason why noise easily superposes on the output signal output from the light receiving element unit is that the fixed slit plate, to which the light receiving element unit is affixed, is made of metal (usually, a metal the same as that of the housing to which said fixed slit plate is affixed). Therefore, the high frequency noise from the circuit for supplying power to the motor, in particular from a circuit controlled by chopping, passes via the case of the motor, the housing, which abuts against an end of the motor case and in which is mounted the bearings supporting the rotary shaft of the motor, to reach the fixed slit plate affixed to the housing. Thus, the fixed slit plate serves as a source of noise.
As a means for reducing the noise, in the prior art, as for example illustrated in FIG. 1, the means has been adopted of connecting a capacitor 104 to the portion (portion of input circuit of comparator 105 in FIG. 1) of the signal processing circuit (comprised of comparator 105, driver 106, etc.) on the printed circuit board 10, to which portion the signal is input from the light receiving element unit 8, where the light receiving elements 82 and 83 are provided, through the connection lines 103, the capacitor 104 absorbing noise included in the signal output from the light receiving element unit 8.
However, with the capacitor 104, both the noise and also the regular output signal are absorbed. In cases, for example, where the motor is rotating at a high speed or where the slits for the signal patterns formed on the rotary encoder disc are narrow (i.e., there are a large number of pulses), when the frequency of the output signal is high, there is a conspicuous rounding of the output signal due to the influence of the capacitor and there is thus the problem of possible failure to obtain a correct signal as the rectangular wave signal on the comparator output side.
Further, even when the fixed slit plate is not made a metal, but of an insulator, about the same thing happens as when it is made of metal, i.e., an electric field created by the high frequency noise is formed through the insulator on the light receiving element unit side. There is a problem in that this type of noise reduction measure is not sufficiently effective.
To solve the above-mentioned problems, the present applicant previously proposed a pulse encoder provided, as shown in FIG. 2 and FIG. 3, with a light emitting unit (for example, a light eitting diode 101), a light receiving element unit 8 for receiving light from the light emitting unit, a rotary encoder disc 9 and a fixed encoder disc 72 having signal patterns which pass or interrupt light sent from the light emitting unit to the light receiving element unit 8, a fixed slit plate 7 to which the fixed encoder disc 72 and the light receiving element unit 8 are affixed, a housing 61 to which the fixed slit plate 7 is affixed, and a printed circuit board 10 on which is formed the signal processing circuit to which signal output from the light receiving element unit 8 is input through connection lines 103. The fixed slit plate 7 is affixed in an electrically insulated state to the housing 61 through, for example, an insulating spacer 71. The fixed slit plate 7 and an earth or ground pattern formed on the light receiving element unit 8 are electrically connected by a fixing screw 82 of the light receiving element unit 8. The ground pattern of the light receiving element unit 8 is electrically connected to the 0.sub.V terminal of the printed circuit board 10 by the connection lines 103 (Japanese Patent Application No. 59-249757, Japanese Unexamined Patent Publication (Kokai) No. 61-128122).
Note that in FIG. 2, and FIG. 3, reference numeral 62 is a shaft of the pulse encoder, which is connected to the motor shaft. The end of the shaft, which is supported by bearings mounted in the housing 61, is shown in a state protruding from the top of the housing. To its top portion 63 is affixed by adhesion the rotary encoder disc 9. Reference numeral 76 is a stopscrew for affixing the fixed slit plate 7 through the insulating spacer 71 to the housing 61. Between the fixed slit plate 7 and the stopscrew 76 is interposed an insulating bushing 75. Reference numerals 102 are fastening screws for fastening the printed circuit board 10 to the housing 61.
According to this construction, the potential of the metal fixed slit plate is stabilized at zero potential. The fixed slit plate thus fractions to electrically "shield" out effects of high frequency noise transmitted from the housing side (absorb lines of electric force flowing in from the housing side). Therefore, the signal output from the light receiving element unit affixed to the fixed slit plate is made free from the high frequency noise. The noise resistance in the pulse encoder could therefore be sufficiently strengthened to enable principal detection of the rotational angle, rotational speed etc. of servo motors to read the position and movement of robot arms, the feed of machine tool work tables, etc.
However, when a motor provided with a pulse encoder such as mentioned above is used for positioning the wire in a wire cut electric discharge machine tool, for example, high frequency noise is created not only by the circuit for supplying power to the motor, but also the current flowing through the wire. There are strong effects by this high frequency noise as well.
Therefore, when the above-mentioned pulse encoder is used for positioning in an electric discharge machine tool, the insulating spacer interposed between the housing and the fixed slit plate must be made considerably thick or else it would be difficult to sufficiently reduce the effects of the high frequency noise in the light receiving element unit side. Further, the effects of the high frequency noise from the portion of the housing not covered by the insulating spacer (for example, the lower half portion of the housing 61 shown in FIG. 3) cannot be neglected.